Cell Division & Reproduction
I. General
Cells divide for three reasons:
1) Growth- increase the number of cells in a multicellular organism
2) Repair- replace damaged cells in a multicellular organism
3) Reproduction- produce new cells for the offspring
II. Two Types of Cell Division
1) Mitosis- nucleus divides producing 2 nuclei each with a full set of DNA (chromosomes)
This requires that the DNA replicate.
This is used for growth, repair and asexual reproduction
2) Meiosis- nucleus divides producing 2 nuclei each with a half set of DNA (chromosomes)
This is used for sexual reproduction
III. Cell Cycle
Interphase- normal growth and function , DNA replicates , cell prepares for mitosis
Mitosis (M Phase) : nucleus divides forming 2 nuclei with full DNA- this occurs in 4 phases
Cytokinesis- cytoplasm divides forming 2 cells
IV. Chromosomes-
Because of replication, each chromosome is made of two identical pieces of DNA- each is called a chromatid and they are attached by a centromere.
While the two chromatids are attached, it is considered to be one chromosome
When the chromatids separate, each is considered to be a chromosome
V. 4 Phases of Mitosis (see lab sheet)
a) Prophase
b) Metaphase
c) Anaphase-
d) Telophase-
VI. Cytokinesis- the cytoplasm divides
plant cells- cell plate
animal cells- pinching in
VII. Asexual Reproduction
One Parent, no joining of cells
Offspring is a clone of the parent- same DNA
Cells for offspring are produced by mitosis
examples- 1)binary fission- one celled organism divides in two
2) Budding- offspring grows on side of parent- eventually separates
3) Vegetative Propagation- parts of plant can grow into a new plant.
VIII. Spore Formation- many organisms reproduce by making cells with a protective coat (spores) These leave the parent and grow if conditions are favorable. Sometimes spore formation is asexual, sometimes sexual.
IX. Sexual Reproduction
A) Special cells join to form a single cell that grows into the offspring
Gametes (sex cells) Fertilization Zygote Development
B) Gametes
1) Gametes have a half set of DNA- zygote has a full set
2) gametes are formed by meiosis
3)Usually two kinds of gametes-
a)male gamete- sperm small, motile produced in the testes
b)female gamete- egg (ovum) larger, stationary- produced in ovaries
4) Hermaphrodite- can produce sperm and egg-
C) Fertilization
Fertilization- Internal (in vivo) external (usually in the water)
external in a "test tube"- in vitro (glass)
self-fertilization cross-fertilization
X. Zygote and Chromosome Number
Chromosomes occur in pairs- homologous pairs- same size and shape- DNA is similar but not identical
Diploid cells- have both members of each pair- The diploid number in humans is 46 Diploid abbrv 2N
Most cells are diploid.
Haploid cells- have one member of each pair- haploid number in humans is 23 Haploid abbrv N
Gametes are haploid When gametes fertilize- the zygote gets 2 members of each pair--> Diploid
Gamete + Gamete ------>Zygote
1/2 set + 1/2 set------>full set
haploid + haploid ------->diploid
one of each pair + one of each pair-------> two of each pair
XI. Meiosis
Meiosis- cell division that produces gametes- produces haploid cells
Spermatogenesis- process that uses meiosis to produce sperm
Oogenesis- process that uses meiosis to produce egg cells.
XI. Development
In multicellular organisms, the zygote divides and develops into the adult organism.
Three main things have to happen-
1) increase the number of cells
2) cells specialize to form different types of cells
3) specialized cells organize into proper body form
The development of most animals follows the same pattern:
Zygote divides over and over- cleavage - 2 celled stage and on -
embryo forms a ball of cells- Blastula stage
A pocket forms inside the ball - gastrula stage
Sometimes an unfertilized egg can develop- Parthenogenesis (ex. male bees)
Some organisms stop developing temporarily at a larva stage which then later changes into the adult-metamorphosis. caterpillar, tadpole
Twins- fraternal- 2 separate zygotes
identical - one zygote- embryo at some point splits